In the prior art, aluminum is well recognized for its corrosion resistance. AA1000 series aluminum alloys are often selected where corrosion resistance is needed.
In applications were higher strengths may be needed, AA1000 series alloys have been replaced with more highly alloyed materials such as the AA3000 series type aluminum alloys. AA3102 is one example of a higher strength aluminum alloy having good corrosion resistance.
Aluminum alloys of the AA3000 series type have found extensive use in the automotive industry due to their combination of high strength, light weight, corrosion resistance and extrudability. These alloys are often made into tubing for use in heat exchanger or air conditioning condenser applications.
One of the problems that AA3000 series alloys have when subjected to corrosive environments is pitting or blistering corrosion. These types of corrosion often occur in the types of environments found in heat exchanger or air conditioning condenser applications and can result in failure of an automotive component where the corrosion compromises the integrity of the aluminum alloy tubing.
In a search for aluminum alloys having improved corrosion resistance, more highly alloyed materials have been developed such as those disclosed in U.S. Pat. Nos. 4,649,087 and 4,828,794. These more highly alloyed materials while providing improved corrosion performance are not conducive to extrusion due to the need for extremely high extrusion forces.
U.S. Pat. No. 5,286,316 discloses an aluminum alloy with both high extrudability and high corrosion resistance. This alloy consists essentially of at least 0.1-0.5% by weight of manganese, about 0.05-0.12% by weight of silicon, about 0.10-0.20% by weight of titanium, about 0.15-0.25% by weight of iron with the balance aluminum. This alloy is essentially copper free with the level of copper not exceeding 0.03% by weight.
Although the alloy disclosed in U.S. Pat. No. 5,286,316 offers improved corrosion resistance over AA3102, even more improved corrosion resistance is desirable. In corrosion testing using salt water-acetic acid sprays as set forth in ASTM Standard G85 (hereinafter SWAAT testing), condenser tubes made of AA3102 material lasted only eight days in a SWAAT test environment before failing. In similar experiments using the alloy taught in U.S. Pat. No. 5,286,316, longer durations than AA3102 were achieved. However, the improved alloy of U.S. Pat. No. 5,286,316 still failed in SWAAT testing in less than 20 days. During this testing, it was discovered that lowering the copper content to impurity levels provided better corrosion resistance than alloy compositions having copper amounts greater than this value. However, maintaining such a low copper content in the aluminum alloy tube to be cast is difficult under industrial casting conditions. Thus, it is impractical to produce an aluminum alloy with such a low copper content in spite of its improved corrosion resistance.
Accordingly, a need has developed to provide an aluminum alloy offering better corrosion resistance, particularly pitting or blistering corrosion resistance, than that provided by AA3102 and the alloy composition disclosed in U.S. Pat. No. 5,286,316. In response to this need, the present invention provides an aluminum alloy material which is more user friendly during manufacture by having practical limitations on the amount of copper while providing improved corrosion resistance over prior art alloys.